Fix conditional type inference involving any or unknown

src/compiler/checker.ts

@@ -10383,11 +10383,11 @@ namespace ts {
             // We attempt to resolve the conditional type only when the check and extends types are non-generic
             if (!checkTypeInstantiable && !maybeTypeOfKind(inferredExtendsType, TypeFlags.Instantiable | TypeFlags.GenericMappedType)) {
                 if (inferredExtendsType.flags & TypeFlags.AnyOrUnknown) {
-                    return trueType;
+                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
                 }
                 // Return union of trueType and falseType for 'any' since it matches anything
                 if (checkType.flags & TypeFlags.Any) {
-                    return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), falseType]);
+                    return getUnionType([combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType, falseType]);
                 }
                 // Return falseType for a definitely false extends check. We check an instantiations of the two
                 // types with type parameters mapped to the wildcard type, the most permissive instantiations
@@ -10402,7 +10402,7 @@ namespace ts {
                 //   type Foo<T extends { x: any }> = T extends { x: string } ? string : number
                 // doesn't immediately resolve to 'string' instead of being deferred.
                 if (isTypeAssignableTo(getRestrictiveInstantiation(checkType), getRestrictiveInstantiation(inferredExtendsType))) {
-                    return instantiateType(root.trueType, combinedMapper || mapper);
+                    return combinedMapper ? instantiateType(root.trueType, combinedMapper) : trueType;
                 }
             }
             // Return a deferred type for a check that is neither definitely true nor definitely false

tests/baselines/reference/inferTypes2.js

@@ -12,6 +12,14 @@ export declare function foo2<T>(obj: T): T extends { [K in keyof BadNested<infer
 export function bar2<T>(obj: T) {
     return foo2(obj);
 }
+
+// Repros from #31099
+
+type Weird = any extends infer U ? U : never;
+type AlsoWeird = unknown extends infer U ? U : never;
+
+const a: Weird = null;
+const b: string = a;
 
 
 //// [inferTypes2.js]
@@ -26,6 +34,8 @@ function bar2(obj) {
     return foo2(obj);
 }
 exports.bar2 = bar2;
+var a = null;
+var b = a;
 
 
 //// [inferTypes2.d.ts]

tests/baselines/reference/inferTypes2.symbols

@@ -53,3 +53,23 @@ export function bar2<T>(obj: T) {
 >obj : Symbol(obj, Decl(inferTypes2.ts, 10, 24))
 }
 
+// Repros from #31099
+
+type Weird = any extends infer U ? U : never;
+>Weird : Symbol(Weird, Decl(inferTypes2.ts, 12, 1))
+>U : Symbol(U, Decl(inferTypes2.ts, 16, 30))
+>U : Symbol(U, Decl(inferTypes2.ts, 16, 30))
+
+type AlsoWeird = unknown extends infer U ? U : never;
+>AlsoWeird : Symbol(AlsoWeird, Decl(inferTypes2.ts, 16, 45))
+>U : Symbol(U, Decl(inferTypes2.ts, 17, 38))
+>U : Symbol(U, Decl(inferTypes2.ts, 17, 38))
+
+const a: Weird = null;
+>a : Symbol(a, Decl(inferTypes2.ts, 19, 5))
+>Weird : Symbol(Weird, Decl(inferTypes2.ts, 12, 1))
+
+const b: string = a;
+>b : Symbol(b, Decl(inferTypes2.ts, 20, 5))
+>a : Symbol(a, Decl(inferTypes2.ts, 19, 5))
+

tests/baselines/reference/inferTypes2.types

@@ -33,3 +33,19 @@ export function bar2<T>(obj: T) {
 >obj : T
 }
 
+// Repros from #31099
+
+type Weird = any extends infer U ? U : never;
+>Weird : any
+
+type AlsoWeird = unknown extends infer U ? U : never;
+>AlsoWeird : unknown
+
+const a: Weird = null;
+>a : any
+>null : null
+
+const b: string = a;
+>b : string
+>a : any
+

tests/cases/conformance/types/conditional/inferTypes2.ts

@@ -14,3 +14,11 @@ export declare function foo2<T>(obj: T): T extends { [K in keyof BadNested<infer
 export function bar2<T>(obj: T) {
     return foo2(obj);
 }
+
+// Repros from #31099
+
+type Weird = any extends infer U ? U : never;
+type AlsoWeird = unknown extends infer U ? U : never;
+
+const a: Weird = null;
+const b: string = a;